Out of all my birds, the only one who's hatch date eluded me were the Meyer's. I guess this is a reality of owning a rehome/rescue bird, you do not always get all the background information.
I was lucky enough that Joey's former owner gave me his hatch certificate when I purchased him; he had got Joey straight from the breeder which meant he got as much information about Joey as possible before he brought him home.
Petey I will most likely never find out his hatch date. The band isn't particularly explicit and I've joined online groups that find out the meaning behind what is on the band and was never successful figuring out what Petey's inscriptions stood for, other than the year of birth. As far as contacting past owners, there are a few and the first I know about wouldn't be much helpful.
Zuri, on the other hand, happened to be hatched as the same aviary as Joey but was sold to a pet store by the breeder, who I happened to know, and his former owner got him there. I also happen to know one of the people working at the pet store, this same person co-runs the rescue from which I got Zuri and Petey, but he couldn't remember when Zuri first arrived at the store. When I first got Zuri, I had tried contacting Zuri's breeder, who no longer breeds bird, asking if she happened to remember when Zuri was hatched, based on the band information. Nothing. After talking to someone who happened to have a bird coming from the same aviary and who got an answer from the breeder for the same thing, I tried again. This time I got an answer.
Turns out Zuri was hatched May 2, 2007. Meaning he turned three at the beginning of the month. I'm very happy I got this information; not that Zuri cares when it's his birthday or not, but I like knowing this information.
So, since it's still May and that he never got his birthday thread, happy belated birthday Zuri :)